President Barack Obama has described Thursday's shooting at a Texas army compound, in which 13, as 'one of the most devastating attacks ever committed on an American military base.'
He cautioned against making hasty assumptions as an investigation was launched into a shooting by a Muslim army psychiatrist.
‘We don't know all of the answers yet. I would caution against jumping to conclusions until we have all of the facts,’ he said.
‘What we do know is that their families, friends, and an entire nation is grieving right now for the valued men and women that came under attack,’ Mr Obama said.
The president led his nation in mourning as shocked Americans struggled to understand why a Muslim army doctor unleashed a massacre at the Fort Hood military base.
Alleged shooter Major Nidal Malik Hasan, 39, a psychiatrist and specialist in combat stress who had been about to deploy to Afghanistan against his wishes, also wounded 30 people in Thursday's deadly rampage.
Speculation swirled at Fort Hood, Texas yesterday as to whether the alleged gunman had snapped under the pressure of his job counselling thousands of war-weary troops, or was motivated by deeper convictions.
President Obama ordered flags to fly at half-staff at the White House and federal buildings, as troops in the US and around the world held a minute's silence to mourn the dead.
Mr Obama would also attend a memorial service due to be held in the coming days, the White House said.
Mr Hasan was moved meanwhile from a civilian to a military hospital, in part for security reasons, Fort Hood deputy commander Colonel John Rossi told reporters.
Army chief of staff General George Casey said the attack was ‘a kick in the gut, not only for the Fort Hood community but for the entire army.’
Mr Hasan was shot and seriously wounded by a female civilian police officer who was being hailed as a heroine for ending his deadly rampage.
Col Rossi said investigators believe Mr Hasan fired more than 100 rounds during the incident, and that that accounted for the high number of casualties.
A surveillance video aired by CNN showed the major buying breakfast wearing traditional Muslim dress at a base store just hours before the shooting.
The bloodshed dealt a new blow to US forces already under severe strain from repeated combat tours and plagued by a rise in suicides and depression.
Fort Hood, by area the world's largest US military base, has borne the brunt of the wars in Iraq and Afghanistan. Troops based here have suffered the highest number of casualties and have undertaken multiple tours of duty.
Amid the sorrow, the shooting raised delicate questions about Muslim soldiers serving in the US Army, as some Muslim groups feared a huge backlash.
Col Casey, the army chief of staff, said after a visit to the base that he, too, feared that possibility. Major Hasan was born in the US to Palestinian parents who had moved from a small town near Jerusalem.
His cousin Nader Hasan, writing on behalf of the family as Mr Hasan's parents are dead, said they were stunned by Thursday's events and stressed they all considered themselves Americans.
‘Our family loves America. We are proud of our country, and saddened by today's tragedy,’ Nader Hasan said in the message posted on The Washington Post website.
Nidal Hasan's aunt, Noel Hasan, told the newspaper her nephew had been subjected to harassment about his faith since the 11 September 2001 attacks and had repeatedly sought to be discharged.
